Heart failure (HF) decompensation is characterized by increased left atrial pressure (LAP) causing symptomatic pulmonary congestion and edema. There may be multiple different catalysts for acute HF, but a common aggravating condition is elevated LAP. The rise in LAP usually is gradual and precedes symptom onset. Accurate monitoring of LAP can enable early diagnosis of incipient decompensation and guide treatment. There are at least three techniques for measuring or estimating LAP. A first technique is direct measurement of LAP using an implantable sensor positioned in the left atrium, as taught in U.S. Pat. No. 6,970,742, entitled, “Method for Detecting, Diagnosing, and Treating Cardiovascular Disease” (Mann et al.), which is incorporated herein by reference. A second technique, widely used in clinical practice, is an estimation of LAP using pulmonary capillary wedge pressure measured by a fluid-filled catheter. A third technique is an indirect estimation of LAP based on estimated intra-thoracic fluid volume changes. This technique relies on the fact that LAP is the primary force governing fluid filtration between the intravascular space and the extravascular space including interstitial space, pulmonary alveolar space, and plural space.
In the third technique described above, LAP is estimated by measuring impedance (Z) between two or more implanted electrodes, for example two or more implantable cardioverter-defibrillator (ICD) or cardiac resynchronization therapy defibrillator (CRT-D) lead electrodes. LAP estimation based on impedance measurements (referred to herein as zLAP) taken between two or more electrodes implanted within the heart is described in U.S. Pat. No. 5,003,976, entitled, “Cardiac and pulmonary physiological analysis via intracardiac measurements with a single sensor” (Alt), which patent document is incorporated herein by reference. Measurement of impedance need not require additional invasive procedures and can potentially offer non-invasive measurements of LAP when a device, such as an ICD or CRT-D, is already implanted. However, impedance measurements are susceptible to drift and/or imprecision due to factors unrelated to LAP, and therefore monitoring based solely on impedance derived LAP may not produce satisfactory results for use in diagnosis and/or treatment. It can be useful to improve the accuracy of zLAP as an estimate for directly measured LAP.
One condition that can result from a failing heart is pulmonary edema. Pulmonary edema is the fluid accumulation in the lungs that may result from elevated LAP over a period of time. Pulmonary edema leads to impaired gas exchange and may cause respiratory failure, and if left untreated can lead to coma and even death due to its main complication of hypoxia. Pulmonary edema may be suspected in the presence of cardiovascular disease and confirmed through physical examination and chest x-ray. In addition, blood tests are typically performed for electrolytes and markers of renal function in order to select an appropriate method of treatment. Usually, by the time a patient presents to the hospital with suspected pulmonary edema, he or she is experiencing symptoms such as shortness of breath with minimal exertion, orthopnea, and swelling of extremities. Prolonging time to treatment increases risk of complications. In addition, a physician examination is subjective and depends on the level of skill and experience of the physician as well as clarity of symptoms. Therefore, it is desirable to improve the timeliness and accuracy of diagnosis to potentially reduce damage caused by such conditions.